1. Field of the Invention
The present invention relates to an electronic part, a method for fabricating the same, an acceleration sensor and a method for fabricating the same. More specifically, the invention relates to a MEMS fabricated by utilizing micro-machining, a method for fabricating the same, an acceleration sensor utilizing the MEMS and a method for fabricating the same.
2. Description of Related Art
MEMS (Micro Electro Mechanical Systems) is a functional part in which mechanical elements and electrical elements are combined. As one example of the MEMS, there is a piezoresistant-type acceleration sensor (see Japanese Patent Application Laid-open No. 03-202778 (JP Patent No. 2127840) for example).
FIG. 17 is substantially a section view of a piezoresistant-type acceleration sensor. When a force caused by acceleration is applied to a dead-weight 82a of the piezoresistant-type acceleration sensor 500, a flexible section 74a deflects and a resistance value of a piezoresistant element (not shown) assembled into the flexible section 74a changes. The changes of current or voltage caused by this change of the resistance value is taken out to the outside from an electric wire (not shown) connected to the piezo resistance via a pad (not shown) formed on an upper surface of an fixing section 76a and a bonding wire 90a electrically connected to that and through an outside wiring electrode 88a. The sensor is enabled to detect the acceleration by detecting this change.
FIG. 18 is an enlarged section view near the pad 92a. The piezo resistances 94a are provided on a surface layer portion of the flexible section 74a and the metal pad 92a is connected with the piezo resistances 94a by an electric wire (not shown). The bonding wire 90a is electrically connected with the metal pad 92a. 
Meanwhile, when ambient temperature of the acceleration sensor changes, thermal stress is generated due to a difference of thermal expansion coefficient of members used in the acceleration sensor, causing erroneous operation.
There has been disclosed a technology of relaxing the thermal stress in Japanese Patent Application Laid-Open No. 2005-337874 for example.
There has been also disclosed a technology of providing resin at a center part of a metal post that forms a part connecting a substrate and an element to relax bonding stress caused in connecting the element provided on the substrate with another substrate by means of a bump and thermal stress caused by the difference of thermal expansion coefficient of those substrates in Republished Patent Nos. WO00/77843 and WO00/77844 for example.
However, even if such function for relaxing the thermal and bonding stress is given, performance of a mechanical element changes in general due to changes of external stress acting on the part of the mechanical element, disabling often to obtain desirable characteristics.
For instance, methods of physically press-bonding by means of ultrasonic and heat are used in connecting the bonding wire 90a to the pad 92a in an acceleration sensor shown in FIG. 18. External stress is applied to a part connecting the bonding wire 90a with the pad 92a when such press-bonding is carried out. Then, there is a problem that the piezoresistant element 94a provided in the flexible section 74a may operate erroneously.
Still more, there is a possibility of deforming or damaging the flexible section 74a in forming the bonding wire 90a on the acceleration sensor due to an impact in connecting the bonding wire 90a to the acceleration sensor for example. There is also a possibility of deforming or damaging the flexible section 74a in the same manner in providing the bump to the acceleration sensor due to an impact in connecting the acceleration sensor to a semiconductor device by the bump.
Still more, there is a problem even if the aforementioned countermeasure for relaxing the thermal stress is taken, tension of the bonding wire 90a in FIG. 17 acts on the flexible section 74a, disabling to correctly detect the acceleration. There is also a problem in directly connecting the piezoresistant-type acceleration sensor 500 with a semiconductor device that stress of a connecting part may act directly on the piezoresistant-type acceleration sensor 500, inducing a deadly characteristic fluctuation.
The problems as described above are actualized when the bonding wire 90a is shortened so that it deflects less in order to accommodate with micronization of electronic part.